The present invention relates to an oscillator circuit, in particular an oscillator circuit of which the oscillating frequency is set by a resistor and a capacitor.
In the application fields of microcomputers, systems have become so complicated that a plurality of microcomputer chips communicate with each other to form one system. Such systems require high-precision reference clocks (e.g., clock signals having no or small jitter) for communication between microcomputer chips. In general, to generate high-precision reference clocks, external components such as quartz oscillators or ceramic oscillators, whose oscillating frequency is substantially unchanged regardless of changes in the environment such as temperature are used. On the other hand, it has been desired to reduce the number of necessary components in order to reduce the total cost of systems and/or to reduce the packaging area.
As a result, it has been proposed to incorporate an oscillator circuit that generates a reference clock into a semiconductor device as a way of reducing the number of external components. As an oscillator circuit embedded in a semiconductor device, an RC oscillator circuit of which the oscillating frequency is set by a resistor and a capacitor is used. Since resistors and capacitors can be easily embedded in semiconductor devices, RC oscillator circuits can generate relatively high-precision clock signals.
U.S. Pat. No. 5,594,388 discloses an example of such oscillator circuits. Therefore, FIG. 9 shows the circuit diagram of an oscillator circuit 100 disclosed in U.S. Pat. No. 5,594,388. The oscillator circuit 100 forms an RC oscillator circuit by using a resistor Rosc108 and a capacitor Cosc109 as base elements. In the oscillator circuit 100, a current Iosc flowing through the resistor Rosc108 is supplied to a ramp-and-hold circuit 130 through a current mirror circuit composed of transistors 111 and 131. Then, the ramp-and-hold circuit 130 charges the capacitor Cosc109 with the current Iosc. In this process, a comparator 120 compares a current that flows to a transistor 124 based on a voltage Vcap generated by the capacitor Cosc with a current that flows to a transistor 122 based on a voltage generated by the resistor Rosc. Then, the direction of the charge/discharge of a capacitor Cpump is determined by a charge pump circuit 170 according to the output of the comparator 120. In this way, an oscillating frequency control voltage Vcp is generated at a node connected to the capacitor Cpump. Then, a voltage control oscillator 180 changes the oscillating frequency of an output signal bout according to the oscillating frequency control voltage Vcp.
Incidentally, in the oscillator circuit 100, the frequency of the output signal output from the voltage control oscillator 180 can be divided by a frequency-dividing circuit 196. Therefore, in the oscillator circuit 100, the comparator 120, and the charge pump circuit 170 are operated at 1/N of the frequency of the output signal.
As described above, in the oscillator circuit 100, instead of using the output of the comparator 120 as the output signal, the charge pump circuit 170 and the voltage control oscillator 180 are controlled based on the output of the comparator 120. By doing so, the oscillating frequency of the output signal can be increased irrespective of the delay time of the comparator 120.